This will be a short topic, but an important one. Object.uc is the highest class in the class tree; all other scripts are subclasses of it. The most important subclass of Object is Actor. When working with UnrealScript, almost all of your work will be under Actor in the class tree. Actor contains code that gives classes a position in the world, lets them easily interact with each other and affect the game in some way. All of the other subclasses of Object can be thought of as more "informational" classes. For instance, if we take a look at InterpTrack and its subclasses, we can see that these classes define the tracks we can use in a Matinee such as movement or animation. The classes themselves have no useful purpose in the game world itself as, say, a projectile would.
Only Actor classes can be spawned, and indeed if we search through Actor.uc we can find the place where that function is declared:
native noexport final function coerce actor Spawn ( class<actor...